The present invention generally concerns spinal implants, and more specifically, but not exclusively, concerns a laterally expandable vertebral implant.
A major cause of persistent, often disabling, back pain can arise by disruption of the disc annulus, chronic inflammation of the disc, or relative instability of vertebral bodies surrounding a given disc, such as might occur due to a degenerative disease. In the more severe cases, some form of mechanical limitation to the movement of the vertebrae on either side of the subject disc is necessary. In such cases, the disc tissue is irreparably damaged, thereby necessitating removal of the entire disc. However, when the disc nucleus is removed without subsequent stabilization the same disabling back pain often reoccurs due to persistent inflammation and/or instability.
Various approaches have been developed to stabilize the adjacent vertebral bodies following excision of this material. In one approach, two adjacent vertebrae are fused together through a fusion device that is implanted between the vertebrae. Many of these existing implant designs have drawbacks that lower the spinal fusion rates. Among these design drawbacks, one such flaw is that the implants subside into the vertebral end plates, thereby reducing the spacing between the vertebral bodies. With prior fusion devices, and even some prosthetic devices, a large portion of the load is placed against the weakest part of the vertebral body, which can lead to cavitation of the device into the surrounding vertebral endplates with subsequent collapse of the inner discal space and even damage of the vertebrae itself. Another frequent cause for subsistence is created by having a small area of contact between the implant and the endplates. As one should appreciate, the less surface area of contact between the implant and the end plates, the greater the risk of subsistence.
Another flaw of many implants is the lack of stability created after implantation. Stability is crucial to the success of a fusion. The implant must be securely fixated to the vertebral bodies in order to ensure that no movement occurs between the two. If movement does occur between the vertebral bodies and the implant, the bone may not properly fuse, thereby creating stability problems. Moreover, some designs limit the amount of graft material, which may be able to be used with the implant. The larger area of graft material that is able to contact the endplates, the better chances of a good, solid bone growth between the two vertebrae.
Some designs have created implants in which the majority of the implant is positioned over the harder cortical bone of the apophyseal ring of the vertebrae in order to reduce the chances of subsistence. However, with these designs, the implant is made from multiple separate components that are individually assembled together within the disc space. Each component is implanted separately and then attached to one another within the disc space. As should be appreciated, assembling such an implant in the disc space can be rather difficult. Such implants also tend to lack a stiff central body, which is essential to the stability of the implant as well as entire fusion construct. Moreover, such implants have no mechanism to fix the implant to the vertebral body. Typically, one has to use bone screws to secure the implant to the vertebral bodies, which makes the implantation process more complicated and difficult. In addition, such implants generally have a single lateral width, and therefore, it is generally very difficult, if not impossible, to adjust for differently sized vertebrae. Another flaw is that these designs typically do not provide a mechanism for ensuring that the spacers are properly positioned. Since the lateral spacers of these types of implants are independently assembled within the disc space, the lateral members can be positioned at unequal positions along the apophyseal ring, thereby increasing the risk that the implant will subside into the vertebral end plates.